Public Release: 9-Feb-1998
Transposable Elements May Have Had A Major Role In The Evolution Of Higher
Organisms

ATHENS, Ga. -- Genes are the on-off switches in plants and animals,
directing everything from growth to fighting disease. Until a mere 50 years ago,
scientists thought all genes worked from a stable position along a chromosome.
Then, a brilliant researcher named Barbara McClintock (who was to win the Nobel
Prize) showed that some genes actually move around.

These genes, which scientists now call transposable elements or
transposons, have been found in vast numbers in virtually every organism
researchers have studied. And yet their role has been the subject of
considerable discussion and even controversy. The question remains: Are
transposable elements merely self-replicating "junk" DNA as some researchers
suspect, or do they contribute to the function and evolution of the organisms in
which they reside?

Now, a molecular biologist at the University of Georgia has proposed
that transposons may play a crucial and central role in evolution and could be
the "missing link" in our understanding of how multicellular and vertebrate
organisms arose.

"The whole idea of transposons as purely selfish DNA is beginning to
crumble," said John McDonald, a professor in the department of genetics at UGA,
"It now appears that at least some transposable elements may be essential to the
organisms in which they reside. Even more interesting is the growing likelihood
that transposable elements have played an essential role in the evolution of
higher organisms, including humans."

McDonald has studied and published papers on transposons for more than
10 years. His
new theory was published in the March issue of the journal Trends in Ecology and
Evolution, which will be released this week.

For years, researchers believed that transposable elements were simply
pieces of rogue DNA, barnstorming through the cellular world like petulant
children, causing the misexpression of other genes and, in general, looking out
for number one. That early picture, however, began to change dramatically in the
1970s, when it became clear that transposons are pervasive. It simply made no
sense that such elements would be conserved over thousands of millennia if they
had no real function. So, a number of scientists, including McDonald, began to
look at the bigger picture, sensing that transposons may be crucially important
to the functioning of all plants and animals.

The evidence is only now becoming clear. Two major events in the history
of life were the origin of eukaryotes (multicellular organisms with well-defined
cell nuclei) and vertebrates. The evolution of these "higher" forms of life
obviously meant a quantum increase in the number of genes needed to direct
cellular functions. A scientist at the University of Edinburgh in Scotland,
Adrian Bird, has argued that in order to keep the genes working in an orderly
manner, higher organisms developed two global gene-silencing mechanisms, one
that aided the transition from bacteria and viruses to more complex organisms
like yeast and invertebrates, and another which smoothed the transition from
invertebrates to vertebrates.

"While Bird's hypothesis is compelling, it did not explain the
evolutionary driving force behind the establishment of global silencing
mechanisms in the first place," said McDonald. "In recent years, a body of
evidence has been accumulating which suggests that these global repression
mechanisms initially arose as a defensive response to the selfish drive of
transposable elements."

The lines of evidence McDonald uses in developing his theory focus on
the two gene-silencing mechanisms that organisms have developed. One of these
mechanisms involves the formation of a tangled complex of DNA and protein called
chromatin within a eukaryotic nucleus. McDonald argues that chromatin formation
likely originated as an adaptive response to the action of transposons.

In fact, studies have found that transposons make up a large part of
constitutive
heterochromatin, regions on chromosomes that are permanently condensed and, for
the most part, genetically inactive in every cell.

The second major gene-silencing mechanism in higher organisms is called
methylation. McDonald points out that the vast majority of methylated gene
sequences in humans and other vertebrates are contained in transposable
elements. Thus, McDonald argues, it is likely that methylation and other related
silencing mechanisms also originally evolved as a defense against transposable
elements.

"By silencing transposable elements, host genes are able to protect
themselves from mutations that actively jumping elements can cause," said
McDonald.

The key lies in understanding that these global-silencing mechanisms,
which evolved in response to "selfish DNA," have been subsequently co-opted by
the host plant or animal cell to serve a regulatory function.

McDonald said it is increasingly clear that organisms need transposons
and that their apparent continual back-door assaults on normal genes may, in
truth, be more like a vast, sophisticated chess game on an enormously complex
board. The new structures and functions that emerge from their battle serve to
drive the evolution of the host genomes in which transposable elements reside.

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